Cooperative Institute for Research in Environmental Sciences

A winning proposal for the Innovative Research Program, 2007:

New Doppler Lidar Using Double-edge Atomic Absorption Filter with 3-Frequency Transmitter to Study Gravity Wave Excitation, Propagation, and Dissipation from Ground to Upper Atmosphere

Investigators: Xinzhao Chu, Wentao Huang, and Mike Hardesty

Objectives: The project is to demonstrate a new lidar technology of using double-edge atomic absorption filters with 3-frequency lidar transmitter to measure wind, temperature and aerosol simultaneously from ground to middle atmosphere. We aim to address a very challenging issue in the weather, climate, and atmospheric chemistry communities – the characterization of small-scale gravity waves and the parameterizations of gravity waves for use in atmospheric general circulation models.

Background and Importance: Atmospheric gravity waves (GW) exist by virtue of the stable density stratification of the atmosphere under gravity. Disturbances to a balanced state can result in excitation of atmospheric gravity waves with a variety of spatial and temporal scales. Gravity waves are important for several reasons: They can transport energy and momentum from one region of the atmosphere to another; they can initiate and modulate convection and subsequent hydrological processes; they disturb the smooth, balanced state through injection of energy and momentum into the flow; and, when the waves break, turbulence hazardous to aviation is generated and chemical species are mixed. Small-scale gravity waves are a key element in defining the large-scale circulation, the thermal and chemical constituent distributions, and the variability of the atmosphere from the troposphere to the lower thermosphere.

The poor representation of gravity waves in modern climate models is a leading source of model uncertainty. Current atmospheric general circulation models cannot resolve gravity waves (usually a few to 100s km) self-consistently because of the coarse model resolution (minimum of 2.5°). Therefore, parameterizations of GWs are needed for use in the models. Owing to our very limited knowledge of GW spectrum, propagation, dissipation, and source distributions, GW parameterizations are currently very poor. Improving GW characterization has been identified by the US and international atmospheric science community as one of the most challenging and urgent issues. This demands high quality measurements of wind and temperature simultaneously for tracking GWs (through wind and temperature perturbations) from their source regions in the troposphere and lower stratosphere to the upper stratosphere, mesosphere and lower thermosphere where GWs dissipate, break, and deposit energy and momentum to the background atmosphere. However, none of current instruments (including radar, lidar, or satellite) is able to make such measurements, due to the limited detection regions of each instrument.

We propose to address this issue through developing an innovative Doppler lidar that can measure temperature and wind simultaneously from ground to mesosphere. This is achieved by using a 3- frequency lidar transmitter with a narrowband lidar receiver armed with double-edge sodium (Na) absorption filters to resolve the Doppler-broadened spectrum of the returned molecular scattering signals. Combining with our existing Na Doppler lidar that is capable of wind and temperature measurements in the mesosphere and lower thermosphere, we can profile the wind and temperature from ground all the way to the middle and upper atmosphere for the first time. This will enable the comprehensive study and improvement of GW characterization and parameterization for use in atmospheric models.

Research Plan: The proposed lidar utilizes the Doppler frequency shift and broadening produced when laser photons are scattered from air molecules in random thermal motion. The Maxwellian distribution of molecular velocities has a width of ~300 m/s that produces Doppler broadening of ~1 GHz. In contrast, aerosols and other particulate matter move with velocities determined by the wind (~10 m/s) and turbulence (~1 m/s) producing Doppler broadening of ~30 MHz and ~3MHz, respectively. As a result, the frequency distribution of light backscattered from the atmosphere consists of a narrow spike near the frequency of the laser transmitter caused by particulate scattering riding on a much broader distribution produced by molecular scattering. By measuring the Doppler shift and broadening of the molecular scattering, the atmosphere wind and temperature can be determined simultaneously.

The proposed double-edge Na absorption filter is composed of a Na vapor cell placed in a strong magnetic field. Lidar returns are selected by a polarizer to a linear polarization and then decomposed to a left and a right circular polarization in the Na vapor cell under a magnetic field. Due to the Zeeman splitting of Na energy levels caused by the strong magnetic field, two circularly polarized lights experience different absorption lines in the Na vapor cell. The two absorption lines act as a double-edge filter with opposite slopes. The transmitted signals passing through the two filters will have different intensities. The difference between the two filtered signals strongly depends on the Doppler frequency shift and broadening. Thus, the ratio of the difference to the sum of the two signals is a sensitive function of the radial wind and temperature. A quarter wave plate, a polarized beam splitter, and two photomultiplier tubes are used to separate and detect the signals from the two filters. With the lidar sequentially transmitting three frequencies produced by an acousto-optic modulator shifting laser frequency up and down, three ratios are obtained for deriving temperature, wind, and aerosol information simultaneously.

We will perform comprehensive quantum mechanics calculation of the Na absorption filter, and then design and build two filters using parts purchased from Mojave Solar Inc. Detailed characterization of the filters will be performed at CIRES lidar laboratory using a narrowband ring dye laser. Once the filters are characterized and calibrated against quantum mechanics calculations, the setup will be installed in the receiver of a 3-frequency resonance fluorescence Na Doppler lidar that Chu and Huang are building at CU. Sky return will be obtained from the NOAA/CU Lidar Observatory at Table Mountain. Since the very low atmosphere returns (below 5 km) involve Brillouin scattering, the resulted pressure broadening will introduce extra complexity into the lidar data analysis. To get better handle on this, the lidar data will be compared against balloon-sonde data and NOAA wind lidar data in the lower atmosphere provided by Hardesty group. Though very challenging technically, the research team has extensive experience and successful track records on lidar technology and laser spectroscopy research, promising high success rate.

What makes this innovative? None of the current lidars can determine wind and temperature simultaneously from ground to mesosphere. They either take temperature and aerosol backscatter ratio profiles from models in deriving radial wind or they assume the vertical wind to be zero in order to derive temperature. The 3-frequency lidar transmitter and the double-edge Na absorption filters proposed here enable us to obtain sufficient spectral information for simultaneous determination of wind, temperature and aerosol profiles. Furthermore, since the natural Na absorption lines are calculated precisely from quantum mechanics and measured precisely with a single frequency laser, the proposed lidar does not have long-term drifting problem that etalon-based lidars suffer, as etalons are environment-sensitive.

How might this be interdisciplinary? Simultaneous wind and temperature measurements are a key issue to both lower and upper atmosphere science communities, especially when concerning the gravity wave excitation, propagation, dissipation and impact to the background atmosphere. It is also a great challenge to the international lidar community. The proposed work will enhance the collaboration between NOAA optical remote sensing group and CIRES lidar group, putting Chu group (mainly middle and upper atmosphere lidar) in direct collaboration with Hardesty group (mainly lower atmosphere and ocean lidar).

Expected Outcome and Impact: This is the first step for lidar and atmospheric science communities to achieve the capability of fully profiling temperature and wind from ground up to 120 km. It will push the observations and studies of gravity waves to a completely new level, helping improve the gravity wave parameterization and reduce climate model uncertainty. It is a proof of concept for the innovative idea of using multiple-frequency lidar transmitter and double-edge atomic absorption filters to measure wind and temperature from troposphere to mesosphere. Once successful, the initial results are a strong driver for a major proposal (~$1M) to the Department of Defense or NSF lower atmosphere division. Furthermore, the proposed work will lead the lidar technology direction that meets the needs of national as well as worldwide researchers. Many scientists on campus expressed such a desire that Chu and her colleagues would extend lidar measurements to the lower atmosphere and earth's surface. The proposed work is the first step to extend the CU lidar programs towards this direction. It will also attract funds internationally.